Dunedin police are investigating an unprovoked and sustained assault outside a party, which has left a teenager in hospital with serious injuries.

A 16-year-old boy was approached by a group of teenagers on Malvern Street, sometime between 10.30pm and midnight last night.

The group of about four to five boys, believed to be between 16 and 18 years old, kicked the victim in the head and body after he fell to the ground.

He was taken to hospital with a badly fractured jaw and suspected fractured ribs.

Detective Sergeant Rob Hanna says police are interested in hearing from people attending a nearby party and also those who witnessed the assault.